Method and apparatus for determining read-head deviation using orthogonal preambles

ABSTRACT

A storage device includes read circuitry having a read head having a detector that outputs signals representing data from a first track and an adjacent track. The read head is subject to off-track excursions during which the read head detects signals from both the first track and an adjacent track. Data on each track includes a preamble including a repeating pattern. The repeating pattern in any first track is orthogonal to the repeating pattern in any track adjacent to the first track. The read circuitry also includes respective Discrete Fourier Transform circuits to identify components in the signals corresponding to respective frequencies characteristic of the repeating pattern on the first track and the repeating pattern on the second track, and computation circuitry to determine from the components a ratio by which the read head is off-track. Corresponding methods are provided for operating such a storage device and for reading data.

CROSS REFERENCE TO RELATED APPLICATION

This claims the benefit of copending, commonly-assigned U.S. ProvisionalPatent Application No. 62/024,251, filed Jul. 14, 2014, which is herebyincorporated by reference herein in its entirety.

FIELD OF USE

This disclosure relates to data storage systems of the type in whichread and write heads move over the surface of a storage medium. Moreparticularly, this disclosure relates to determining the degree ofoff-track deviation of a read head, to improve read performance.

BACKGROUND

The background description provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of theinventors hereof, to the extent the work is described in this backgroundsection, as well as aspects of the description that may not otherwisequalify as prior art at the time of filing, are neither expressly norimpliedly admitted to be prior art against the present disclosure.

In magnetic recording, as one example, reading and writing are performedby one or more heads that move relative to the surface of a storagemedium. Many magnetic disk drives, for example, include a plurality ofindividual disks, or “platters,” which may be two-sided—i.e., eachplatter can store data on each of its two sides. Therefore, such a diskdrive would have at least two heads for each platter. Indeed, for eachplatter, there is normally at least one write head and at least oneseparate read head, so that such a disk drive normally has at least fourheads per platter.

In a common configuration, all of the heads in a given disk drive aremounted on arms attached to a common actuator that controls the radialposition of the heads (an angular, tangential or circumferentialcomponent of motion is provided by the rotation of the platters relativeto the heads). This is true whether there is one or many platters, andone or multiple heads per platter.

In order to control the radial position selected by the actuator, eachsurface of each platter has distributed upon it positional informationreferred to as “servo” data. The servo data are commonly distributed inspaced-apart servo “wedges” (generally spaced equiangularly) on theplatter surface. By reading the servo data as each servo wedge passesunder the read head, the disk drive controller can determine the preciseradial and angular position of the head and can feed back thatdetermination to control the position of the read head or the writehead, depending on the required operation. Among the servo data are syncmarks, which are used to determine angular position. Separate instancesof the sync mark are provided at different radial positions—i.e., fordifferent tracks.

Data on adjacent tracks are generally independent of one another.However, as areal densities for magnetic data storage continue toincrease, data tracks are being written in an overlapping or “shingled”fashion—e.g., in “two-dimensional magnetic recording” (TDMR)—and readingis performed using read heads or sensors having dimensions comparable tothe track width. Therefore, it is important to know whether, and by howmuch, a read head is deviating from its nominal position relative to atrack.

SUMMARY

A method according to this disclosure, for operating a storage devicehaving a storage medium, includes writing data to tracks on the storagemedium, where data for each track including a preamble including arepeating pattern, and wherein the repeating pattern in any first trackis orthogonal to the repeating pattern in any track adjacent to thefirst track. The method further includes detecting with a read headsignals from both the first track and an adjacent track, and analyzingpreamble output of the read head to determine a ratio by which the readhead is off-track.

In a further implementation of that method, the analyzing includesapplying a Discrete Fourier Transform to the preamble output of the readhead.

According to another implementation, a method of reading data from astorage device having a storage medium to which data are written intracks, where data on each track includes a preamble including arepeating pattern, and where the repeating pattern in any first track isorthogonal to the repeating pattern in any track adjacent to the firsttrack, includes detecting the first track with a read head subject tooff-track excursions during which the read head detects signals fromboth the first track and an adjacent track, and analyzing preambleoutput of the read head to determine a ratio by which the read head isoff-track.

According to a further implementation of that method, the analyzingincludes applying a Discrete Fourier Transform to the preamble output ofthe read head.

An implementation of a storage device according to this disclosureincludes read circuitry having a read head having a detector thatoutputs signals representing data from a first track and an adjacenttrack, where the read head is subject to off-track excursions duringwhich the read head detects signals from both the first track and anadjacent track, and where data on each track including a preambleincluding a repeating pattern, with the repeating pattern in any firsttrack being orthogonal to the repeating pattern in any track adjacent tothe first track. The read circuitry also includes respective DiscreteFourier Transform circuits to identify components in the signalscorresponding to respective frequencies characteristic of the repeatingpattern on the first track and the repeating pattern on the secondtrack, and computation circuitry to determine from the components aratio by which the read head is off-track.

According to a further implementation of that storage device, thedetector outputs analog signals, and the read circuitry further includesanalog-to-digital converter circuitry that outputs samples of the analogsignals for input to the respective Discrete Fourier Transform circuits.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features of the disclosure, its nature and various advantages,will be apparent upon consideration of the following detaileddescription, taken in conjunction with the accompanying drawings, inwhich like reference characters refer to like parts throughout, and inwhich:

FIG. 1 is a side elevational view of a portion of a disk drive withwhich the present disclosure may be used;

FIG. 2 is a plan view of the disk drive portion of

FIG. 1, taken from line 2-2 of FIG. 1;

FIG. 3 is a schematic representation of an example of adjacent datatracks with an off-track read head;

FIG. 4 is a schematic representation of a controller for the drive ofFIGS. 1 and 2;

FIG. 5 is a schematic representation of off-track detection circuitrythat may be used in apparatus according to this disclosure;

FIG. 6 is a graph of Discrete Fourier Transform results in animplementation of a method according to this disclosure; and

FIG. 7 is a flow diagram of an implementation of an alternate methodaccording to this disclosure.

DETAILED DESCRIPTION

FIGS. 1 and 2 show an example of a disk drive 100 with which the presentdisclosure may be used. In this particular example, disk drive 100 hasthree platters 101, 102, 103, although any number of platters may beincluded in a disk drive with which the present disclosure may be used.As shown, each platter 101, 102, 103 has, on each of its upper and lowersurfaces 111, 112, a coating 110 made from a material in which data canbe stored, e.g., magnetically. The present disclosure also is relevantto a disk drive in which one or more platters includes coating 110 ononly one of its surfaces, but such a disk drive would store less data inthe same volume than a disk drive with two-sided platters. The platters101-103 are mounted on a rotatable spindle 104. Motor 105 rotatesspindle 104 to rotate platters 101-103 in the direction of arrow A (FIG.2). Although motor 105 is shown connected directly to spindle 104, insome cases motor 105 may be located off-axis of spindle 104 and would beconnected to spindle 104 through belts or gears (not shown).

Read/write head assembly 120 includes an actuator 121 that bears arms122-125, one of which is disposed adjacent to each surface 111, 112 of aplatter 101, 102, 103 that has a memory storage coating 110. In thisexample, with heads on both surfaces of each of arms 123, 124, thatamounts to four arms 122-125, but in the single-sided platter examplediscussed above, there would be only three arms. In other examples, thenumber of arms would increase or decrease along with the number ofplatters.

Each arm 122-125 bears, at or near its end furthest from actuator 121,and on both its upper and lower surfaces in the case of arms 123, 124, aplurality of read heads/sensors and write heads. In this case, twosensors 131, 132 are shown, and will be used to represent read sensors,although it would normally at least be expected that each set of one ormore read sensors has a companion write head (not shown). It should benoted that FIGS. 1 and 2 are schematic only and not to scale. Forexample, the spindle diameter may be larger by comparison to the diskdiameter.

A motor 126, commonly referred to as a “voice-coil motor,” rotatesactuator 121 back and forth along the directions of arrow B (FIG. 2) tomove the heads 131, 132 along the path indicated by dashed arrow 201,although arms 122-125 normally cannot point directly at the center ofthe disk. The motion of actuator 121 thus changes both the radial andcircumferential positions of heads 131, 132, but the circumferentialpositional change is relatively unimportant insofar as the platters arerotating. The motion of actuator 121 thus is used to control the radialposition of heads 131, 132.

The location on surface 111 of platter 101 (the other surfaces aresimilar) of the aforementioned wedges is shown in FIG. 2. Each servowedge 200 includes data identifying it by wedge, or sector, number (togive an angular, tangential or circumferential position) and by datarepresenting, at each point along a radius of the platter, the distancefrom spindle 104, although sometimes some of this information is omittedfrom some of the wedges.

As noted above, as areal densities for magnetic data storage continue toincrease, data tracks are being written in an overlapping or “shingled”fashion—e.g., in “two-dimensional magnetic recording” (TDMR)—and readingis being performed using read heads or sensors having dimensionscomparable to the track width. Therefore, it is important to knowwhether, and by how much, a read head is deviating from its nominalposition relative to a track. If an off-track condition is detected,system parameters can be optimized for better performance based on theamount by which the heads are off-track. For example, filter parameters,as well as the amount of gain for each head, can be adjusted based onthe amount by which the heads are off-track.

In accordance with implementations of this disclosure, adjacent tracksmay be written with orthogonal preambles—i.e., preambles that aremutually exclusive, as described in copending, commonly-assigned U.S.patent application Ser. No. 14/563,578, filed Dec. 8, 2014, which ishereby incorporated by reference herein in its entirety. Specifically,the preambles may be chosen to be a pair of periodic patterns such thatthe inner product of the two patterns in question, when represented assequences of write current polarities {−1,1}, over a window chosen tocontain an integer number of periods of both patterns, is zero. Forexample, a 2T pattern (110011001100 . . . ) may be written to thepreambles on each even-numbered track, while a 3T pattern(111000111000111000 . . . ) may be written to the preambles on eachodd-numbered track, although any pair of periods where one period is nota multiple of the other may be selected. There may be other patterns aswell, such as a 4T pattern (111100001111000011110000 . . . ). Any pairof such orthogonal patterns may be used for adjacent tracks inaccordance with this disclosure. These patterns may be described interms of tones (i.e., sinusoidal signals at the fundamental frequenciesof the respective patterns).

A pair of tracks 301, 302 with orthogonal preambles 311, 312 asdescribed above is shown in FIG. 3. Although tracks 301, 302 are shownare shown as being straight, in the case of a rotating medium such as adisk drive platter, tracks 301, 302 actually would be curved. FIG. 3also shows a read head 300 which is in an off-track condition; althoughread head 300 nominally should be aligned with track 301, read head 300as shown is mainly over track 301, but is partially over track 302 aswell. The portions of tracks 301, 302 that are shown would be outsidethe servo wedges 200 —i.e., preambles 311, 312 are interspersed amongthe user data outside the servo wedges 200 (e.g., at the beginnings ofat least some data packets). Normally, each preamble is followed by oneof sync marks 321, 322. However, in some cases, extra preambles may beinserted without sync marks.

Read head 300 is connected to a read channel 401 of a hard drivecontroller 400 (FIG. 4). In addition, an unseen write head is connectedto a write channel 402 of hard drive controller 400. Hard drivecontroller 400 also includes a processor 410 and memory 411, as well asa connection 412 to a host processor (not shown). Memory 411 may be usedto store the position error signal (PES) data that indicates the trackposition offsets. A servo control loop in hard drive controller 400 usesthe PES data to keep read head 300 (as well as the unseen write head) ontrack.

In accordance with an implementation of this disclosure, read channel401 of hard drive controller 400 also includes Discrete FourierTransform (DFT) circuitry 500, shown in more detail in FIG. 5, todetermine the amount by which read head 300 is off-track. Read channel401 will include, as is well known, an analog front end (AFE) 413 thatwill output mainly sinusoidal signals. In an example where theorthogonal preambles 311, 312 of the two tracks 301, 302 include,respectively, a 2T tone and a 3T tone, the sinusoidal signal derived byAFE 413 from the 2T tone will have a period of 4T (1100), while thesinusoidal signal derived from the 3T tone will have a period of 6T(111000). If other tones are used, they will have corresponding periods.

The two sinusoidal signals from AFE 413 are input to analog-to-digitalconverter (ADC) 502 to provide digitized ADC samples 512. Digitized ADCsamples 512 are then filtered by finite-impulse-response (FIR) filter503 to provided FIR samples 513. A multiplexer 504 selects eitherdigitized ADC samples 512, or FIR samples 513.

First, as described above, the tones correspond to sinusoidal signals atfrequencies determined by the respective patterns. For some tones (e.g.,3T and 4T), the DFT process may reveal a fundamental tone and additionalharmonics. Once the harmonics are identified, the portion of each signalattributable to the fundamental frequency of each tone can bedetermined.

Multiplexer output samples 514 are input to Discrete Fourier Transformcircuit 505 operating at the first (e.g., 2T) frequency and to DiscreteFourier Transform circuit 506 operating at the second (e.g., 3T)frequency. In Discrete Fourier Transform (DFT) circuits 505, 506, p₁ isthe period of the first frequency, p₂ is the period of the secondfrequency, and N is the number of samples to be accumulated. Each DFTcircuit has a multiplier 515 or 516 that convolves each of N sampleswith sin(2πt/p₂) or sin(2πt/p₂), respectively, and a multiplier 525 or526 that convolves each of the N samples with cos(2πt/p₂) orcos(2πt/p₂), respectively. Respective accumulators 535, 536 accumulatethe multiplication results over all samples.

Discrete Fourier Transform circuit 505 outputs signals s₁ and c₁,representing the sine and cosine of the contribution of the first toneto the output of read head 300, while Discrete Fourier Transform circuit506 outputs signals s₂ and c₂, representing the sine and cosine of thecontribution of the second tone to the output of read head 300. Thesesignals s₁, c₁, s₂, c₂ can be used in computation block 507 (discussedbelow) to determine a ratio representing how much of read head 300 isover track 301 and how much of read head 300 is over track 302.

FIG. 6 shows the DFT results for 2T and 3T tones for an example in whichthe FIR filter equalizes the channel to the target response of[4,7,1,0,0]. As can be seen, each of the 2T tones 601 (all tones arefundamental) has a magnitude A_(2T)=21.540659228538015. The 3T tone hasboth fundamental (602) and harmonic (612) components; the fundamentalcomponent 602 of the 3T tone has a magnitude A_(3T)=26.263620804789610.

Returning to FIG. 5, block 507 can compute the magnitudes of thecontributions to the samples 514 by the fundamental components of eachof the 2T (all components are fundamental) and 3T tones, which may berepresented as m_(2T) and m_(3T). For an N-point DFT:

$m_{2T} = {\frac{2}{N} \cdot \sqrt{s_{1}^{2} + c_{1}^{2}}}$$m_{3T} = {\frac{2}{N} \cdot \sqrt{s_{2}^{2} + c_{2}^{2}}}$

The desired off-track ratio r may then be computed as follows:

r=(m_(2T)/A_(2T))/(m_(3T)/A_(2T))

The DFT computations in block 507 can be performed by suitablearithmetic circuits (e.g., arithmetic logic units (ALUs)) that performthe arithmetic and trigonometric functions needed. Alternatively,however, block 507 could be implemented as one or more look-up tableswith pre-computed results, where the values of s₁, c₁, s₂, c₂ are usedas addresses into the look-up table to extract the correct results.

As one example, a look-up table implementation can be configured as atwo-stage operation. In a first stage, m_(2T) and m_(3T) can be derivedfrom a look-up table using s₁, c₁, s₂, c₂ as inputs. Because theexpressions for m_(2T) and m_(3T) are the same (see above), this stagecan be further streamlined by using the same look-up table for both them_(2T) calculation and the m_(3T) calculation. This can be done eitherwith a dual-port look-up table, or by time-division multiplexing asingle-port look-up table, although the latter option would be slower.Or two separate look-up tables can be used, one of which determinesm_(2T) from s₁, c₁, and the other of which determines m_(3T) from s₂,c₂. Either way, the second stage can use the m_(2T) and m_(3T) resultsas addresses into another look-up table that provides the value of theratio r. Alternatively, a single look-up table can be used to derive rdirectly from s₁, c₁, s₂, c₂, but because different combinations of s₁,c₁, s₂, c₂ may yield the same value of r, such an alternative may beless efficient because it may require a larger look-up table.

Once the ratio r has been computed based on reading signals frompreambles 311, 312, then as is well-known, r can be used when readinguser data to determine how much of the read signals are attributable toeach of data payloads 331, 332, and thereby to determine those datapayloads 331, 332.

A method 700 of operating a storage device according to this disclosureis diagrammed in FIG. 7. In a method including writing data, at 701,data are written to tracks on the storage medium, including a preamblefor each track, where the preamble for any track is orthogonal to thepreamble for any adjacent track. At 702, the read head is positionedover a target track to be read. At 703, read head signals are detected.As noted above, the read head signals are likely to containcontributions from both the target track and an adjacent track. At 704,preamble output of the read head is analyzed, as described above, todetermine a ratio by which the read head is off-track. At 705, the ratiois used to identify and read data from target track. At 706, it isdetermined whether there are any additional tracks to be read.

If there are additional tracks to be read, method 700 returns to 702 toread those tracks. If at 706 there are no additional tracks to be read,method 700 ends. Note that a method of reading tracks that havepreviously been written would begin at 702 rather than at 701, asindicated by dashed line 707.

It will be understood that the foregoing is only illustrative of theprinciples of the invention, and that the invention can be practiced byother than the described embodiments, which are presented for purposesof illustration and not of limitation, and the present invention islimited only by the claims which follow.

What is claimed is:
 1. A method of operating a storage device having astorage medium, the method comprising: writing data to tracks on thestorage medium, data for each track including a preamble including arepeating pattern, wherein the repeating pattern in any first track isorthogonal to the repeating pattern in any track adjacent to the firsttrack; detecting with a read head signals from both the first track andan adjacent track; analyzing preamble output of the read head todetermine a ratio by which the read head is off-track.
 2. The method ofclaim 1 wherein the analyzing comprises applying a Discrete FourierTransform to the preamble output of the read head.
 3. The method ofclaim 2 further comprising filtering the preamble output of the readhead prior to the applying.
 4. The method of claim 2 wherein theapplying comprises applying a separate Discrete Fourier Transform for afrequency corresponding to the repeating pattern on the first track anda frequency corresponding to the repeating pattern on the adjacenttrack.
 5. The method of claim 4 wherein the analyzing comprises derivinga detected magnitude and a fundamental frequency magnitude for eachfrequency from outputs of the Discrete Fourier Transforms.
 6. The methodof claim 5 further comprising: identifying fundamental and harmoniccomponents prior to the deriving; and performing the deriving on onlythe fundamental components.
 7. A method of reading data from a storagedevice having a storage medium to which data are written in tracks, withdata on each track including a preamble including a repeating pattern,wherein the repeating pattern in any first track is orthogonal to therepeating pattern in any track adjacent to the first track, the methodcomprising: detecting the first track with a read head subject tooff-track excursions during which the read head detects signals fromboth the first track and an adjacent track; analyzing preamble output ofthe read head to determine a ratio by which the read head is off-track.8. The method of claim 7 wherein the analyzing comprises applying aDiscrete Fourier Transform to the preamble output of the read head. 9.The method of claim 8 further comprising filtering the preamble outputof the read head prior to the applying.
 10. The method of claim 8wherein the applying comprises applying a separate Discrete FourierTransform for a frequency corresponding to the repeating pattern on thefirst track and a frequency corresponding to the repeating pattern onthe adjacent track.
 11. The method of claim 10 wherein the analyzingcomprises deriving a detected magnitude and a fundamental frequencymagnitude for each frequency from outputs of the Discrete FourierTransforms.
 12. The method of claim 11 further comprising: identifyingfundamental and harmonic components prior to the deriving; andperforming the deriving on only the fundamental components.
 13. Astorage device comprising: read circuitry including: a read head havinga detector that outputs signals representing data from a first track andan adjacent track, the read head being subject to off-track excursionsduring which the read head detects signals from both the first track andan adjacent track, data on each track including a preamble including arepeating pattern, wherein the repeating pattern in any first track isorthogonal to the repeating pattern in any track adjacent to the firsttrack; respective Discrete Fourier Transform circuits to identifycomponents in the signals corresponding to respective frequenciescharacteristic of the repeating pattern on the first track and therepeating pattern on the second track; and computation circuitry todetermine from the components a ratio by which the read head isoff-track.
 14. The storage device of claim 13 wherein: the detectoroutputs analog signals; and the read circuitry further comprisesanalog-to-digital converter circuitry that outputs samples of the analogsignals for input to the respective Discrete Fourier Transform circuits.15. The storage device of claim 14 further comprising filter circuitrybetween the analog-to-digital converter circuitry and the respectiveDiscrete Fourier Transform circuits.
 16. The storage device of claim 15wherein the filter circuitry comprises a finite impulse response filter.17. The storage device of claim 14 wherein the computation circuitryderives a detected magnitude and a fundamental frequency magnitude foreach frequency from outputs of the Discrete Fourier Transform circuits.18. The storage device of claim 17 wherein: the computation circuitrycomprises a look-up table; and the outputs of the Discrete FourierTransform circuits are addresses into the look-up table.
 19. The storagedevice of claim 18 further comprising filter circuitry on outputs of theanalog-to-digital converter circuitry; wherein: the computationcircuitry operates also on output from the filter circuitry.
 20. Thestorage device of claim 19 wherein the filter circuitry comprises finiteimpulse response filter circuitry.